On Fri, 23 Sep 2016 14:42:53 +0800
"Hillf Danton" <hillf...@alibaba-inc.com> wrote:
> >
> > The select(2) syscall performs a kmalloc(size, GFP_KERNEL) where size grows
> > with the number of fds passed. We had a customer report page allocation
> > failures of order-4 for this allocation. This is a costly order, so it might
> > easily fail, as the VM expects such allocation to have a lower-order
> > fallback.
> >
> > Such trivial fallback is vmalloc(), as the memory doesn't have to be
> > physically contiguous. Also the allocation is temporary for the duration of
> > the
> > syscall, so it's unlikely to stress vmalloc too much.
> >
> > Note that the poll(2) syscall seems to use a linked list of order-0 pages,
> > so
> > it doesn't need this kind of fallback.
How about something like this? (untested)
Eric isn't wrong about vmalloc sucking :)
Thanks,
Nick
---
fs/select.c | 57 +++++++++++++++++++++++++++++++++++++++++++--------------
1 file changed, 43 insertions(+), 14 deletions(-)
diff --git a/fs/select.c b/fs/select.c
index 8ed9da5..3b4834c 100644
--- a/fs/select.c
+++ b/fs/select.c
@@ -555,6 +555,7 @@ int core_sys_select(int n, fd_set __user *inp, fd_set
__user *outp,
void *bits;
int ret, max_fds;
unsigned int size;
+ size_t nr_bytes;
struct fdtable *fdt;
/* Allocate small arguments on the stack to save memory and be faster */
long stack_fds[SELECT_STACK_ALLOC/sizeof(long)];
@@ -576,21 +577,39 @@ int core_sys_select(int n, fd_set __user *inp, fd_set
__user *outp,
* since we used fdset we need to allocate memory in units of
* long-words.
*/
- size = FDS_BYTES(n);
+ ret = -ENOMEM;
bits = stack_fds;
- if (size > sizeof(stack_fds) / 6) {
- /* Not enough space in on-stack array; must use kmalloc */
+ size = FDS_BYTES(n);
+ nr_bytes = 6 * size;
+
+ if (unlikely(nr_bytes > PAGE_SIZE)) {
+ /* Avoid multi-page allocation if possible */
ret = -ENOMEM;
- bits = kmalloc(6 * size, GFP_KERNEL);
- if (!bits)
- goto out_nofds;
+ fds.in = kmalloc(size, GFP_KERNEL);
+ fds.out = kmalloc(size, GFP_KERNEL);
+ fds.ex = kmalloc(size, GFP_KERNEL);
+ fds.res_in = kmalloc(size, GFP_KERNEL);
+ fds.res_out = kmalloc(size, GFP_KERNEL);
+ fds.res_ex = kmalloc(size, GFP_KERNEL);
+
+ if (!(fds.in && fds.out && fds.ex &&
+ fds.res_in && fds.res_out && fds.res_ex))
+ goto out;
+ } else {
+ if (nr_bytes > sizeof(stack_fds)) {
+ /* Not enough space in on-stack array */
+ if (nr_bytes > PAGE_SIZE * 2)
+ bits = kmalloc(nr_bytes, GFP_KERNEL);
+ if (!bits)
+ goto out_nofds;
+ }
+ fds.in = bits;
+ fds.out = bits + size;
+ fds.ex = bits + 2*size;
+ fds.res_in = bits + 3*size;
+ fds.res_out = bits + 4*size;
+ fds.res_ex = bits + 5*size;
}
- fds.in = bits;
- fds.out = bits + size;
- fds.ex = bits + 2*size;
- fds.res_in = bits + 3*size;
- fds.res_out = bits + 4*size;
- fds.res_ex = bits + 5*size;
if ((ret = get_fd_set(n, inp, fds.in)) ||
(ret = get_fd_set(n, outp, fds.out)) ||
@@ -617,8 +636,18 @@ int core_sys_select(int n, fd_set __user *inp, fd_set
__user *outp,
ret = -EFAULT;
out:
- if (bits != stack_fds)
- kfree(bits);
+ if (unlikely(nr_bytes > PAGE_SIZE)) {
+ kfree(fds.in);
+ kfree(fds.out);
+ kfree(fds.ex);
+ kfree(fds.res_in);
+ kfree(fds.res_out);
+ kfree(fds.res_ex);
+ } else {
+ if (bits != stack_fds)
+ kfree(bits);
+ }
+
out_nofds:
return ret;
}
--
2.9.3
